The bug of `docker exec` at zsh completions.

[ktrysmt]

Description
I found a bug at zsh completions.
Completion fails with docker exec.

Steps to reproduce the issue:

• Environment: zsh
• Type to docker exec [TAB]

Describe the results you received:

• Appeared that;

docker exec __docker_container_subcommand:161: command not found: (--help -):containers:__docker_complete_running_containers
__docker_container_subcommand:161: command not found: (--help -):containers:__docker_complete_running_containers
__docker_container_subcommand:161: command not found: (--help -):containers:__docker_complete_running_containers

Describe the results you expected:
(I want that to work correctly.)

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client:
 Version:      17.09.1-ce
 API version:  1.32
 Go version:   go1.8.3
 Git commit:   19e2cf6
 Built:        Thu Dec  7 22:22:25 2017
 OS/Arch:      darwin/amd64

Server:
 Version:      17.09.1-ce
 API version:  1.32 (minimum version 1.12)
 Go version:   go1.8.3
 Git commit:   19e2cf6
 Built:        Thu Dec  7 22:28:28 2017
 OS/Arch:      linux/amd64
 Experimental: true

Output of docker info:

No need it.

Additional environment details (AWS, VirtualBox, physical, etc.):